This invention relates generally to navigation systems and more specifically to a system for positioning radiosondes, sonobuoys, aircraft, ships, land vehicles, and other objects on or near the earth's surface using satellites of the Global Positioning System (GPS). The GPS is a multiple-satellite based radio positioning system in which each GPS satellite transmits data that allows a user to precisely measure the distance from selected ones of the GPS satellites to his antenna and to thereafter compute position, velocity, and time parameters to a high degree of accuracy, using known triangulation techniques. The signals provided by the GPS can be received both globally and continuously. The GPS comprises three major segments, known as the space, control, and user segments.
The space segment, when fully operational, will consist of twenty-one operational satellites and three spares. These satellites will be positioned in a constellation such that typically seven, but a minimum of four, satellites will be observable by a user anywhere on or near the earth's surface. Each satellite transmits signals on two frequencies known as L1 (1575.42 MHz) and L2 (1227.6 MHz), using spread spectrum techniques that employ two types of spreading functions. C/A and P pseudo random noise (PRN) codes are transmitted on frequency L1, and P code only is tranmitted on frequency L2. The C/A or coarse/acquisition code, is available to any user, military or civilian, but the P code is only available to authorized military and civilian users. Both P and C/A codes contain data that enable a receiver to determine the range between a satellite and the user. Superimposed on both the P and C/A codes is the navigation (Nav) message. The Nav message contains 1) GPS system time; 2) a handover word used in connection with the transition from C/A code to P code tracking; 3) ephemeris data for the particular satellites being tracked; 4) almanac data for all of the satellites in the constellation, including information regarding satellite health, coefficients for the ionospheric delay model for C/A code users, and coefficients used to calculate universal coordinated time (UTC).
The control segment comprises a master control station (MCS) and a number of monitor stations. The monitor stations passively track all GPS satellites in view, collecting ranging data and satellite clock data from each satellite. This information is passed on to the MCS where the satellites' future ephemeris and clock drift are predicted. Updated ephemeris and clock data are uploaded to each satellite for re-transmission in each satellite's navigation message. The purpose of the control segment is to ensure that the information transmitted from the satellites is as accurate as possible.
GPS is intended to be used in a wide variety of applications, including space, air, sea, and land object navigation, precise positioning, time transfer, attitude reference, surveying, etc. GPS will be used by a variety of civilian and military organizations all over the world. A number of prior art GPS receivers have been developed to meet the needs of the diverse group of users. These prior art GPS receivers are of a number of different types, including sequential tracking, continuous reception, multiplex, all in view, time transfer, and surveying receivers.
A GPS receiver comprises a number of subsystems, including an antenna assembly, an RF assembly, and a GPS processor assembly. The antenna assembly receives the L-band GPS signal and amplifies it prior to insertion into the RF assembly.
The RF assembly mixes the L-band GPS signal down to a convenient IF frequency. Using various known techniques, the PRN code modulating the L-band signal is tracked through code-correlation to measure the time of transmission of the signals from the saellite. The doppler shift of the received L-band signal is also measured through a carrier tracking loop. The code correlation and carrier tracking function can be performed using either analog or digital processing.
The control of the code and carrier tracking loops is provided by the GPS processor assembly. By differencing this measurement with the time of reception, as determined by the receiver's clock, the pseudo range between the receiver and the satellite being tracked may be determined. This pseudo range includes both the range to the satellite and the offset of the receiver's clock from the GPS master time reference. The pseudo range measurements and navigation data from four satellites are used to compute a three dimensional position and velocity fix, to calibrate the receiver's clock offset, and to provide an indication of GPS time.
The receiver processor controller (RPC) processing and memory functions performed by a typical GPS receiver include monitoring channel status arid control, signal acquisition and reacquisition, code and carrier tracking loops, computing pseudo range (PR) and delta range (DR) measurements, determining data edge timing, acquisition and storage of almanac and ephemeris data broadcast by the satellites, processor control and timing, address and command decoding, timed interrupt generation, interrupt acknowledgment control, and GPS timing, for example. These functions are fixed point operations and do not require a floating point coprocessor.
The navigation processing and memory functions performed by a typical GPS receiver include satellite orbit calculations and satellite selection, atmospheric delay correction calculations, navigation solution computation, clock bias and rate estimates, computation of output information, and preprocessing and coordinate conversion of aiding information, for example. These functions require significant amounts of processing and memory and are generally performed using a floating point coprocessor.
The GPS standard positioning service provides a navigation accuracy of 100 m 2 dRMS. A number of applications of the GPS require higher levels of accuracy. Accuracy can be improved using a technique known as differential GPS (DGPS). This technique involves operating a GPS receiver in a known location. The receiver is used to compute satellite pseudo range correction data using prior knowledge of the correct satellite pseudo ranges, which are then broadcast to users in the same geographic area. The pseudo range corrections are incorporated into the navigation solution of another GPS receiver to correct the observed satellite pseudo range measurements, thereby improving the accuracy of the position determination. Correlation of the errors experienced at the reference station and at the user location is dependent on the distance between them, but they are normally highly correlated for a user within 350 kilometers of the reference station.
An alternative to the GPS receiver known in the prior art is the GPS translator or transdigitizer, as described in U.S. Pat. No. 4,622,557, for example. These translators or transdigitizers typically include only the antenna assembly and RF assembly portions of a GPS receiver. Translators are typically employed in missile tracking applications where small, lightweight, expendable sensors are required. The GPS C/A code spread spectrum signals received by the translator are combined with a pilot carrier and transmitted at S-band frequencies (2200 to 2400 MHz). A GPS translator processor located at the telemetry tracking site receives these translated GPS C/A code signals and estimates the position and velocity of the object. The transdigitizer retransmits the digitally sampled GPS signal at 2 Msps using quadraphase modulation at 149 to 170 MHz.
Known variants of the GPS translator are the digital translator and the transdigitizer. An object-borne GPS digital translator or transdigitizer operates to convert the GPS C/A code spread spectrum signals to base band and perform in-phase and quadrature phase sampling at a rate of about 2 MHz. Transdigitized or translated GPS signals are processed in a ground based translator processing system in a similar manner to GPS signals.
A third variant of the GPS translator is the codeless GPS receiver, as typified by the teachings of U.S. Pat. No. 4,754,283. This receiver ignores the bi-phase code and recovers the carrier frequency of all satellites in view of the receiving antenna. A telemetry transmitter transmits a signal that contains the GPS carrier frequency information to a ground-based telemetry receiver. This data is used to derive the speed of the sonde. Since the GPS code is not tracked, the position of the sonde cannot be computed using this method. This system uses a telemetry link at 403 MHz with a bandwidth of 20 KHz and has the advantage of requiring less bandwidth than the transdigitizer but the disadvantage of only providing velocity data instead of both position and velocity data.
In summary, prior art GPS receivers may be one of three types. In the first type, all navigation processing activities occur at the receiver, which outputs the position and velocity of the tracked object using either a single computer or an RPC and navigation computer, in which there is substantial interconnection between the RPC functions and the navigation functions for satellite selection and acquisition. In the second type of GPS receiver, the GPS signal is remoted by translation or variations thereof and the signal is tracked at a ground processing facility where the object position and velocity are derived. In accordance with this latter approach, significant bandwidth is required to transmit the translated signal. In the third type, the carrier frequency of the GPS signals is measured and retransmitted to the ground processing facility where only the velocity of the object can be derived.
It is therefore the principal object of the present invention to provide a low cost tracking system for radiosondes, sonobuoys, aircraft, ships, land vehicles, and other objects, using GPS satellites, that is capable of providing the position and velocity of multiple objects without requiring a 2 MHz bandwidth data link.
This and other objects are accomplished in accordance with the illustrated preferred embodiment of the present invention by providing a GPS sensor module that supplies the data required to locate a particular object, a one-way telemetry link, and a data processing workstation to process the data and display the object position and velocity. The GPS sensor module comprises an antenna and a sensor. The sensor operates autonomously following application of operating power. The sensor digitally samples the signals from visible GPS satellites and stores this data in a digital buffer. No processing functions are performed by the sensor, thereby permitting significant reductions in the cost thereof. The raw satellite data stored in the buffer, interleaved with other telemetry data from the sonde or other object, are transmitted back to the data processing workstation. Using this set of raw satellite data, the position and velocity of the sensor can be determined at the time the data was recorded by the sensor to a precision of 100 meters. If differential corrections are also provided at the data processing workstation, the accuracy of the position fix can be improved to better than 10 meters. If a 20 kHz data link is used and the GPS signals are sampled at 2 Mbps, a 1-second set of GPS data can be provided every 100 seconds, or a 0.5-second set of GPS data every 50 seconds, or a 0.1-second set of data every 10 seconds. The principal advantage afforded by the present invention is its ability to provide extremely accurate position, velocity, and time information for radiosondes, sonobuoys, and other objects using a low cost sensor and a conventional data telemetry link. By eliminating all processing functions performed in prior art GPS sensors, significant cost reductions are achieved over existing GPS receiver designs. By reducing the data link bandwidth from the 2 MHz required of prior art transdigitizers, conventional telemetry links may be employed to retransmit the data. For low cost data applications, such as sonobuoys or radiosondes, a position and velocity fix is only required at a low rate (e.g. every 10 seconds), a requirement that is accomodated by the present invention.